This invention relates to a gas sensor employed in an exhaust gas system for an internal combustion engine of an automotive vehicle, for example, utilized for combustion control of the internal combustion engine.
The exhaust gas system for an internal combustion engine is usually equipped with a gas sensor. Combustion control of the internal combustion engine is performed based on a sensing signal of the gas sensor so as to enhance the efficiency of exhaust gas purification.
In general, the gas sensor has an activation temperature. The capability of generating an accurate sensing signal is only effected when the temperature of the gas sensor exceeds the activation temperature. Hence, a heater is generally incorporated in the gas sensor to obtain an accurate sensing value as early as possible in the startup stage of the internal combustion engine.
According to recent enhancement of exhaust gas regulations, gas sensors are strongly required to increase their warm-up abilities compared with those of conventional gas sensors.
For example, to realize prompt activation, it is effective to increase a heat generation amount of the heater so as to shorten a time required for the gas sensor to reach its activation temperature.
As a method for increasing the heat generation amount of the heater, it may be effective to select a material having a small electric resistive value for a heat generating section to be accommodated into the heater.
However, excessively increased heat generation will induce large thermal shock in the heater, represented by generation of cracks. Furthermore, it is known that excessive heat generation induces early deterioration of the heater. In view of the foregoing, merely increasing the heat generation amount of the heater is not desirable to realize the prompt activation.
In view of the foregoing problems of the prior art, the present invention has an object to provide a gas sensor capable of realizing prompt activation without causing adverse effects of heat generation, such as thermal shock.
To accomplish the above and other related objects, the present invention provides a first gas sensor comprising a gas sensing element including a cup-shaped cylindrical solid electrolytic element having a reference gas chamber defined therein, a measured gas sensing electrode provided on an outer surface of the solid electrolytic element, and a reference gas sensing electrode provided on an inner surface of the solid electrolytic element facing said reference gas chamber, and a heater accommodated in the reference gas chamber. According to the first gas sensor, a contact portion is provided on an outer cylindrical surface of the heater so that the contact portion is brought into contact with an inside surface of the reference gas chamber, and a heat generating peak position of the heater is in the vicinity of the contact portion.
The most remarkable feature of the first gas sensor of the present invention is that the heat generating peak position of the heater is in the vicinity of the contact portion where the outer cylindrical surface of the heater is brought into contact with the inside surface of the reference gas chamber.
The present invention characterized by the above-described features operates in the following manner.
According to the first gas sensor of the present invention, the heat generating peak position of the heater is in the vicinity of the contact portion.
The contact portion is a portion where the heater is brought into contact with the inside surface of the solid electrolytic element. In other words, the contact portion is a portion where thermal energy of the heater is most effectively transferred to the gas sensing element.
Accordingly, even if the heat generation amount of the heater is the same as that of a conventional heater, generated heat of the heater can be effectively used to warm up the gas sensing element. Thus, it becomes possible to realize prompt activation without adverse effects of heat generation or thermal shock including deterioration of the gas sensing element and the heater.
The heat generating peak position of the heater is a position where the temperature of the heater is highest as shown in FIG. 4.
Thus, according to the first gas sensor of the present invention, it becomes possible to realize prompt activation without causing adverse effects of heat generation, such as thermal shock.
According to the first gas sensor in accordance with the present invention, a heater as shown in FIG. 3 comprises a ceramic core rod and a ceramic sheet wound around this core rod. In general, a heat generating section and a lead section made of an electrically conductive paste are printed on the ceramic sheet.
The components of the electrically conductive paste constituting the heat-generating portion are W, Wxe2x80x94Mo, Wxe2x80x94Re, Pt etc.
The contact portion provided on the outer cylindrical surface of the heater, as shown in FIG. 1, may be an annular portion formed at the distal end of the heater which is coaxially disposed with respect to the gas sensing element in the reference gas chamber. Alternatively, it is preferable that, as shown in FIG. 10, the contact portion may be a local spot on the outer surface of the heater.
The first gas sensor of the present invention can be used for various purposes, such as combustion control of an internal combustion engine. Furthermore, the present invention is widely applicable to all of the sensors which are equipped with a heater placed in an inside space of a sensor element.
Next, the present invention provides a second gas sensor comprising a gas sensing element including a cup-shaped cylindrical solid electrolytic element having a reference gas chamber defined therein, a measured gas sensing electrode provided on an outer surface of the solid electrolytic element, and a reference gas sensing electrode provided on an inner surface of the solid electrolytic element facing said reference gas chamber, and a heater accommodated in the reference gas chamber. According to the second gas sensor, the heater has a heat generating section for generating heat in response to electric power supplied thereto, and an electric resistive value of the heat generating section is maximized in the vicinity of the contact portion (refer to later-described FIG. 9) where the heater is brought into the gas sensing element.
With this arrangement, the heat generation amount increases in the vicinity of the contact portion of the heater. The gas sensing element can be effectively heated. The activation time can be shortened.
Accordingly, even if the heat generation amount of the heater is the same as that of a conventional heater, the generated heat of the heater can be effectively used to warm up the gas sensing element. Thus, it becomes possible to realize the prompt activation without adverse effects of heat generation or thermal shock including deterioration of the gas sensing element and the heater.
Thus, according to the second gas sensor of the present invention, it becomes possible to provide a gas sensor capable of realizing prompt activation without causing adverse effects of heat generation, such as thermal shock.
To form the portion of a ceramic heater where the electric resistive value r is maximized, as shown in later-described FIG. 4A, there is a method for increasing the electric resistance by partly narrowing a line width of the heat generating section at a portion corresponding to the heat generating peak position of the heater.
Furthermore, there is another method of increasing the electric resistance by partly thinning a thickness of the heat generating section at a portion corresponding to the heat generating peak position of the heater.
Furthermore, there is another method of increasing the electric resistance by partly using a different material having a high electric resistance for the heat generating section at a portion corresponding to the heat generating peak position of the heater (refer to later-described FIG. 9).
Next, the present invention provides a third gas sensor comprising a gas sensing element including a cup-shaped cylindrical solid electrolytic element having a reference gas chamber defined therein, a measured gas sensing electrode provided on an outer surface of the solid electrolytic element, and a reference gas sensing electrode provided on an inner surface of the solid electrolytic element facing the reference gas chamber, and a heater accommodated in the reference gas chamber. According to the third gas sensor, a heat generating section of the heater has a heat line pattern whose density is maximized in the vicinity of a contact portion (refer to later-described FIG. 11) where the heater is brought into said gas sensing element.
With this arrangement, the heat generation density increases in the vicinity of the contact portion of the heater. The gas sensing element can be effectively heated. The activation time can be shortened.
Accordingly, even if the heat generation amount of the heater is the same as that of a conventional heater, the generated heat of the heater can be effectively used to warm up the gas sensing element. Thus, it becomes possible to realize the prompt activation without adverse effects of heat generation or thermal shock including deterioration of the gas sensing element and the heater.
Thus, according to the third gas sensor of the present invention, it becomes possible to realize prompt activation without causing adverse effects of heat generation, such as thermal shock.
To maximize the pattern density (i.e., density of a heater line pattern) of the heat generating section of the ceramic heater as described above, there is a method for forming a heat generation section in a concentrated manner to form the heat generating peak position, for example, as shown in later-described FIG. 11.
Next, the present invention provides a fourth gas sensor comprising a gas sensing element including a cup-shaped cylindrical solid electrolytic element having a reference gas chamber defined therein, a measured gas sensing electrode provided on an outer surface of the solid electrolytic element, and a reference gas sensing electrode provided on an inner surface of the solid electrolytic element facing the reference gas chamber, and a heater accommodated in the reference gas chamber. According to the fourth gas sensor, the heater has a heat generating section for generating heat in response to electric power supplied thereto, and the heat generating section has a high resistive portion provided closer to a proximal end of the gas sensor.
Providing the high resistive portion of the heat generating section closer to the proximal end of the gas sensor in this manner makes it possible to moderate the heat generating peak so as not to cause sudden increase as shown in FIG. 12. It becomes possible to suppress the temperature increase in the vicinity of the heat generating peak. An overall temperature distribution becomes uniform.
Thus, according to the fourth gas sensor of the present invention, it becomes possible to realize prompt activation without causing adverse effects of heat generation, such as thermal shock.
Next, the present invention provides a fifth gas sensor comprising a gas sensing element including a cup-shaped cylindrical solid electrolytic element having a reference gas chamber defined therein, a measured gas sensing electrode provided on an outer surface of the solid electrolytic element, and a reference gas sensing electrode provided on an inner surface of the solid electrolytic element facing the reference gas chamber, and a heater accommodated in the reference gas chamber. According to the fifth gas sensor, the heater has a heat generating section for generating heat in response to electric power supplied thereto. A contact portion is provided on an outer cylindrical surface of the heater so that the contact portion is brought into contact with an inside surface of the reference gas chamber. And, a heat generating peak position of the heater appears within xc2xe of a line segment extending between a distal end of a heat generating pattern closer to the contact portion and a center of the heat generating pattern for more than one fifth of a time required for the heat generating peak position of the heater to reach 900xc2x0 C.
Providing the heat generating portion satisfying the above conditions makes it possible to effectively use the generated heat of the heater to warm up the gas sensing element even if the heat generation amount of the heater is the same as that of a conventional heater. Thus, it becomes possible to realize the prompt activation without adverse effects of heat generation or thermal shock including deterioration of the gas sensing element and the heater.
If the heat generating peak position exists within the above xc2xe line segment region for a short duration less than one fifth of the time required for the heat generating peak position to reach 900xc2x0 C., the activation of the gas sensing element will be delayed.
Furthermore, it is desirable that the heat generating peak position remains within the above xc2xe line segment region until the heat generating peak position reaches 900xc2x0 C.
Furthermore, if the heat generating peak position is offset toward the center of the heat generating pattern out of the above xc2xe line segment region for more than one fifth of the time required for reaching 900xc2x0 C., the activation of the gas sensing element will be delayed.
Thus, according to the fifth gas sensor of the present invention, it becomes possible to realize prompt activation without causing adverse effects of heat generation, such as thermal shock.